Electrical resistance in metals arises because electrons that are propagating through the solid are scattered because of deviations from perfect translational symmetry. These deviations are produced either by impurities or the phonon lattice vibrations. The impurities form the temperature independent contribution to the resistance, and the vibrations form the temperature dependent contribution.
Electrical resistance, in some applications, is very undesirable. For example, in electrical power transmission, electrical resistance causes power dissipation, i.e. loss. The power dissipation grows in proportion to the square of the current, namely P=I2R in normal wires. Thus, wires carrying large currents dissipate large amounts of energy. Moreover, the longer the wire used in either larger transformers, bigger motors or larger transmission distances, the more dissipation, since the resistance in a wire is proportional to its length. Thus, as wire lengths increase more energy is lost in the wires, even with a relatively small currents. Consequently, electric power plants produce more energy than that which is used by consumers, since a portion of the energy is lost due to wire resistance.
In a superconductor that is cooled below its transition temperature TC, there is no resistance because the scattering mechanisms are unable to impede the motion of the current carriers. The current is carried, in most known classes of superconductor materials, by pairs of electrons known as Cooper pairs. The mechanism by which two negatively charged electrons are bound together is described by the BCS (Bardeen Cooper Schrieffer) theory. In the superconducting state, i.e. below TC, the binding energy of a pair of electrons causes the opening of a gap in the energy spectrum at Ef, which is the Fermi energy or the highest occupied level in a solid. This separates the pair states from the “normal” single electron states. The size of a Cooper pair is given by the coherence length which is typically 1000 Å, although it can be as small as 30 Å in the copper oxides. The space occupied by one pair contains many other pairs, which forms a complex interdependence of the occupancy of the pair states. Thus, there is insufficient thermal energy to scatter the pairs, as reversing the direction of travel of one electron in the pair requires the destruction of the pair and many other pairs due to the complex interdependence. Consequently, the pairs carry current unimpeded. For further information on superconductor theory please see “Introduction to Superconductivity,” by M. Tinkham, McGraw-Hill, New York, 1975.)
Many different materials can become superconductors when their temperature is cooled below TC. For example, some classical type I superconductors (along with their respective TC's in degrees Kelvin (K)) are carbon 15K, lead 7.2K, lanthanum 4.9K, tantalum 4.47K, and mercury 4.47K. Some type II superconductors, which are part of the new class of high temperature superconductors (along with their respective TC's in degrees K), are Hg0.8Tl0.2Ba2Ca2Cu3O8.33 138K, Bi2Sr2Ca2Cu3O10 118 k, and YBa2Cu3O7-x 93K. The last superconductor is also well known as YBCO superconductor, for its components, namely Yttrium, Barium, Copper, and Oxygen, and is regarded as the highest performance and highest stability high temperature superconductor, especially for electric power applications. YBCO has a Perovskite structure. This structure has a complex layering of the atoms in the metal oxide structure. FIG. 1 depicts the structure for YBa2Cu3O7, that include Yttrium atoms 101, Barium atoms 102, Copper atoms 103, and Oxygen atoms 104. For further information on oxide superconductors please see “Oxide Superconductors”, Robert J. Cava, J. Am. Ceram. Soc., volume 83, number 1, pages 5-28, 2000.
A problem with YBCO superconductors specifically, and the oxide superconductors in general, is that they are hard to manufacture because of their oxide properties, and are challenging to produce in superconducting form because of their complex atomic structures. The smallest defect in the structure, e.g. a disordering of atomic structure or a change in chemical composition, can ruin or significantly degrade their superconducting properties. Defects may arise from many sources, e.g. impurities, wrong material concentration, wrong material phase, wrong processing temperature, poor atomic structure, and improper delivery of materials to the substrate, among others.
Thin film YBCO superconductors can be fabricated in many ways including pulsed laser deposition, sputtering, metal organic deposition, physical vapor deposition, and chemical vapor deposition. Two typical ways for the deposition of thin film YBCO superconductors are described here as example. In the first way, the YBCO is formed on a wafer substrate in a reaction chamber 200, as shown in FIG. 2 by metal organic chemical vapor deposition (MOCVD). This manner of fabrication is similar to that of semiconductor devices. The wafer substrate is placed on holder 201. The substrate is heated by heater 202. The wafer substrate is also rotated which allows for more uniform deposition on the substrate wafer, as well as more even heating of the substrate. Material, in the form of a gas, is delivered to the substrate by shower head 203, via inlet 204. The shower head 203 provides a laminar flow of the material onto the substrate wafer. The material collects on the heated wafer substrate to grow the superconductor. Excess material is removed from the chamber 200 via exhaust port 208, which is coupled to a pump. To prevent undesired deposition of material onto the walls of the chamber 200, coolant flows through jackets 205 in the walls. To prevent material build up inside the shower head 203, coolant flows through coils 206 in the shower head. The flanges port 207 allows access to the inside of the chamber 200 for insertion and removal of the film/substrate sample. Processing of the film may be monitored through optical port 209.
In the second way, YBCO is formed by pulsed laser deposition on a substrate, including the possibility of using a continuous metal tape substrate 301. The tape substrate 301 is supported by two rollers 302, 303 inside of a reaction chamber 300. Roller 302 includes a heater 304, which heats the tape 301 up to a temperature that allows YBCO growth. The material 305 is vaporized in a plume from a YBCO target by irradiation of the target by typically an excimer laser 306. The vapor in the plume then forms the YBCO superconductor film on the substrate 301. The rollers 302, 303 allow for continuous motion of the tape past the laser target thus allowing for continuous coating of the YBCO material onto the tape. Note that the laser 306 is external to the chamber 300 and the beam from the laser 306 enters the chamber 300 via optical port 307. The resulting tape is then cut, and forms a tape or ribbon that has a layer of YBCO superconductive material.
Neither of the above described methods for forming thin film high temperature superconductors can produce a long length tape or ribbon of YBCO which can be used to replace copper (or other metal) wires in electric power applications. The first way only allows for the production of small pieces of superconductor material on the wafer, e.g. a batch process. The second way can only be used to make tape that is a few feet in length and uses multiple passes to generate a superconducting film of several microns thickness. The second way has a practical limitation of about 5 feet. Larger pieces of tape would require a larger heating chamber. A larger heating roller will also be needed. The tape will cool down after leaving roller 302, and thus will need more time to heat back up to the required temperature. Heating on one side of the chamber, with a cool down on the other side of the chamber may also induce thermal cracks into the YBCO layer and other layers formed on the metal substrate. The smaller pieces of tape produced by the second method may be spliced together to form a long length tape, but while the pieces may be superconducting, splice technology is not yet at the point of yielding high quality high temperature superconductor splices. Consequently, current arrangements for forming superconductors cannot form a long, continuous tape of superconductor material.